1. Field of the Invention
The present invention relates to a communication device, and particularly to a communication device for performing fault detection on a network.
2. Description of the Related Art
The physical line of one port connecting two communication devices is constituted by two lines, which serve a bidirectional link between the two communication devices. There are cases when a fault occurs with one link of the two links (called “unidirectional link fault”). With the unidirectional link fault, one of the communication devices is possible to send messages but not to receive, while the other of the communication devices is possible to receive messages but not to send, whereby various types of faults such as looping occur on the network. This kind of fault can be resolved by detecting the unidirectional link fault and closing the port for which the fault was detected.
A method for detecting the aforementioned unidirectional link fault, a UDLD (Uni-Directional Link Detection) protocol has been proposed. (See, for example, Cisco System UniDirectional Link (UDLD) Protocol. <URL:http://www.ietf.org/internet-drafts/draft-foshciano-udld-03.txt/>.)
With the unidirectional link fault detection using this UDLD, first and second communication devices are sending and receiving link normal confirmation frames, and the first device judges the state of the link from the first communication device toward the second communication device based on the link normal notification frame sent from the second device to the first device.
However, with the unidirectional link fault detection using the aforementioned UDLD, typically, the fault detection time is long, and the time required from occurrence of a fault for closing the fault occurring port is considerably long, so there is the problem that the fault status is maintained for a long time. Because of this, there is a desire to shorten the unidirectional link fault detection time.